How to wire two 12V transformers together to get 6V?

[dimmreaper]

I've got a couple of .33KVA 12V transformers kicking around and want to build a psu for a 172W TEC. While one of these transformers has more more than enough wattage to run the TEC, getting the appropriate voltage, without building a step-up switching supply isn't possible AFAIK with a single transformer.

Here is what I'm thinking wire the two together to get 6V (I'm pretty sure it can be done), then using two capicitors and two diods(configured in a "voltage doubling" circuit) to give me two times the peak voltage (peak voltage figures out to be about 10V with the two transformers wired together). This should give me a 20V 330W PSU.

Problem is I'm not possitive on how to wire the two together, and my radioshack book doesn't tell me how.

I figure that wiring the two transformers primaries in series, and the secondaries in parrallel would give me the proper effect. Essentially that should make the two transformers in to one transformer with with half the turn ratio, right? Does anyone know if ths is the case?

I know some of you guys are HAMs so help me out on this!

[dimmreaper]

Here is what I had in mind, will it work? The file is a 9k black and white BMP, it'll download quick, I promise.

[Naeleros]

I see two butterflies kissing...

Oh wait.. maybe its the Mona Lisa and she's giving me the finger?

Sorry.. I'm a totally dunce for electronics.. but.. I had to let you know the first things that popped into my head when I saw the picture :)

James

[Quaky]

I think you will have major difficulties with that setup, problem is that the transformers need to be exactly the same. If they aren't (and even 2 transformers of the same type and brand are hardly ever the same) they will get different input voltages, and thus different output voltages.

Image if transfo A has a higer output voltage than transfo B: Due to the difference in potential you will see a current flowing from A to B, this means that you will overpower transfo B and you will have backflow at transfo B. This situation is not good (you propably guessed that already :p) and will seriously reduce the effectiveness of the setup. I don't know for sure, but I think that the life span of the transfos will be reduced as well.

OK, enough about the problem: here 's the solution...
Hook up the transfo's in parallel, primary and secondary coils! Make sure you check the polarity prior to connecting the secondaries! After that apply the voltage doubler (note that you will need heavy caps since you are pulling a lot of current) you will then have +/- 35 volts DC. Without the voltage doubler you get 17.5 which might be enough (depends on your pelt) Lowering the voltage kan be done with a simple light dimmer in the primaries. With the dimmer you have full control over the pelts voltage as a bonus ;)

I hope this helps you... And if you actually try it can you send me results? :D

[cjtune]

I hope you have power circuits to properly 'smoothen' the DC output from your transformers. Simply running a rectified DC pulse and running a pelt based on its Vrms is no good. The repeating cycles of high and low voltage (that although sums up to the require Vrms) will give lesser than expected performance due to a combination of electrical heating (over Vmax) and undercooling (under Vmax) as well as subject the peltier elements to cyclic stress -fatigue failure.

I can't load your BMP file for some reason...

[JigPu]

Your schematic looks as though you are hooking a DC source to the transformers. Did you do this just to help us figure out the polarities, or did you not know that transformers only work on AC?? I hope it was the first one.

[begin internal mind monolouge to help me out]
As for hooking the transformers like that, you are doubling the primary and keeping the secondary the same. Therefore, 1/2 the ratio bringing the output from the transformers to 6V AC. Don't now the formula for RMS to peak, so I'll assume peak is 10V. Voltage doubler brings it to 20V...
[end internal mind monolouge to help me out]

OK! The only problem I see is you will be feeding pure AC to the pelts (unless the voltage doubler also rectifies). That is bad. Set up a recitifier after the doubler to change the AC to pulsing DC. Now, filter that down to just DC.

Hope it helps,
JigPu

[dimmreaper]

To answer the questions regarding to input and output. The input will be 110VAC, and the output is fully rectified and filtered 20VDC. The two diods perform the rectification, the two capacitors would perform the filtering as well as voltage doubling.

However, the Idea of using a dimmer switch on the input had not occured to me. If I can find a dimmer that can handle the amperage, it would eliminate the need to use the second transformer. Then I could just use one transformer with the voltage doubler.

Thanx for all the input guys!

[Quaky]

I have a dimmer here rated to 800watts. I believe there are even more powerfull dimmers out there, so finding one won't be a problem...